Polymerization inhibitor



2,964,131 POLYMERIZATION INHIBITOR Dan N. Campbell, Texas City, Tex., assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed Jan. 2, 1959, Ser. No. 784,468 9 Claims. (Cl. 183-115) This invention relates to the inhibition of polymerization. More particularly, it relates to the inhibition of undesirable polymer growth in processing equipment during the purification of acetylene by absorption techniques.

When acetylene is produced from hydrocarbons, e.g., by the well known electric arc process, by thermal decomposition or dissociation, or by partial oxidation, the acetylene-containing gaseous mixture produced generally contains so-called poly-formers, that is, compounds of the nature of biacetylene, for example, which polymerize to solid materials under certain conditions of temperature, pressure, etc. The purification of such an acetylenic gas mixture is usually accomplished by the selective action of suitable solvents. A number and variety of solvents have been proposed among which may be mentioned, for example, water, acetone, dimethylformamide, butyrolactone, N-methyl pyrrolidone, triethyl phosphate, and diethyl carbonate. The normal procedure for separation and recovery of acetylene from gas mixtures by means of absorption in a preferential liquid solvent comprises absorbing the acetylene in the solvent in an absorption tower, removing those gases which are slightly soluble in the solvent in a stripping tower and then subjecting the acetylene solution to desorption which is eflected by means of increased temperatures, decreased pressures, or a combination of both. However, in these conventional methods of absorption and stripping the gas mixtures is exposed to conditions under which the poly-formers mentioned above polymerize and provision must be made in all such processes for periodic removal of the polymeric substances to prevent their excessive build-up in the solvent in the absorption and stripping steps. This requires additional, specialized filtration equipment as well as added process steps which are costly with respect to both time and capital expense. Frequently, too, the polymeric material collects in the less accessible parts of the system, fouling up equipment lines, columns and the like. Usually, too, the polymeric material as formed is of such a small particle size that it plugs the filter used for separating it, so that only a small amount can be filtered in each operation and an inordinate length of time is required to wring the filter cake dry.

Methods have been proposed for removal of certain of the known poly-formers from the gas stream, but these are cumbersome and expensive since they require additional process steps prior to the usual solvent purification operation which utilize special equipment and significant quantities of special chemicals for treating purposes.

It is an object of the present invention, therefore, to provide a method whereby undesirable polymer growth during the solvent purification of acetylene is inhibited. Other objects and advantages of the invention will be apparent from the following description and the appended claims.

According to the invention the formation of polymers in an acetylene purification system wherein a solvent is employed for the selective absorption of acetylene from an acetylene-containing gas stream derived from hydrocarbons is eftectively inhibited by incorporating in said solvent a minor amount of an amine oxide or an amine .oxide hydrate.

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The invention is illustrated in the following examples which, however, are not to be construed as limiting it in any manner.

EXAMPLE I The testing apparatus consisted of a series of Pyrex gas-washing bottles (500 ml. capacity) connected in parallel to gas-inlet and outlet manifolds. The distributing outlets on the gas-inlet manifold all contained steel needle valves and rotameters for regulating and measuring the gas flow. A series of of tests were conducted in which certain of the bottles in a group were charged with acetylene solvent, N-methyl pyrrolidone, alone while others of the group were filled with this same solvent containing varying amounts of amine oxides or amine oxide hydrates. The same procedure was followed in each set of tests. The solvent was carefully weighed into each bottle so that the same amount, 100 g. (approximately 100 ml.) was used in each one. An acetylenecontaining gas stream was bubbled through the liquid in the bottles at a fixed rate of about 600 ml. per min. for a period of 24 hours. The gas employed was the eflluent stream produced by the oxidation of methane in a suitable reactor. It was tapped from a process line in an acetylene plant at a point downstream from the furnace or reactor after the gas had been quenched and filtered to remove any soot present. This gas was then successively passed through a compressor, a pressure regulator, and a chilled surge container prior to admittance into the gas-inlet manifold.

After the bubblers had been in operation for 24 hours, the bottles were disconnected and the solutions were weighed. The contents were then transferred to clean glass bottles and analyzed to determine the amount of polymer present. Polymer determinations were made as follows: Ten-ml. samples of the solutions were pipetted into 65-ml. ignition tubes. The solutions were frozen solid by immersing the tubes in a trichloroethylene-Dry Ice bath and the tubes were then sealed by means of an oxygen-natural gas torch. After allowing them to warm up to room temperature, the tubes were placed in an oven maintained at 130 C. for min. At the end of this time the tubes were removed, allowed to cool to "room temperature, then further cooled to solidify the contents again by freezing, and opened. The opened tubes were allowed to warm up to room temperature after which the contents of each were poured into separate Erlenmeyer flasks. The tubes were washed with 0.1 N H and the washings were added to the respective flasks. The flasks were heated on a hot plate until the mixtures therein were brought to a near boil. The contents of each flask were then filtered on separate tared Gooch crucibles, the flasks being rinsed with hot water and the washings being poured onto the filters. The crucibles were dried in a vacuum oven at C. for 1 hour, cooled in a desiccator and weighed to determine the amount of polymer in each sample.

Results of the tests are presented in Table I. The data recorded demonstrate that acetylene-containing gas produced from hydrocarbons contains polymerizable compounds which polymerize under conditions such as are used in a process for purifying or recovering acetylene from such gases by absorption in a solvent having preferential solvent action for acetylene. The data also convincingly demonstrate that such polymerization is inhibited if an amine oxide or a hydrate of an amine oxide is incorporated in the solvent. Decreases in polymer production ranging from 22% to 63% have been realized using the amine oxides or their hydrates.

EXAMPLE II A gas mixture obtained by the partial combustion of methane with oxygen in a suitable furnace containing the following constituents in mol percent is compressed and fed under pressure into the middle of an absorption tower. This is a tower containing trays, pans, or ferrules of any suitable type. The gaseous mixture ascends into the upper portion of the tower coming into contact therein with descending liquid N-methyl pyrrolidone containing in solution about 1.0% by weight of N,N-diethyl-2-aminoethanol-N-oxide hydrate admitted at the top of the tower. As the two streams of ascending vapor and descending liquid come into contact, the acetylene in the gas dissolves in the N-rnethyl pyrrolidone so that by the time the liquid solvent reaches the bottom or the absorber it is saturated with acetylene. It contains also small amounts of the other volatile constituents which are slightly soluble such as carbon dioxide and ethylene. An overhead gas essentially free of acetylene is withdrawn from the top of the absorber.

Table I Percent iAmt'. Polymer in Inhibitor, Inhibited Inhibitor Wt. Solvent] Sample Compound Percent of Percent Percent No. Added Solvent Polymer Polymer in Uninhibited Solvent 22-6 None None 1. 14 22-2 Triethanolamine- 1.0 0.57 0. 50

N-oxide hydrate. 22-3 N,N-diethyl-2- 1.0 0.47 0.41

aminoethanol- N-oxide hydrate. 24-1 None None 1. 35 24-7 N ,N-diethyl-2- 0. 1.06 0.78

aminoethauol- N-oxide hydrate. 24-8 Triethanolamine- 0. 1 1.19 0.88

N-oxide hydrate. 26-1 None None 1.19 26-1 1(2-hydroxy- 1.0 0.77 0.65

ethyD-Z-methyl- Q-imidazoline- N -oxide. 28-5-13-.-" None None 1. 44 28-1 N-methylmor- 1. 0.90 0.63

pholine-N- oxide. N-ethylmorpho- 1. 1. 04 0. 72

line-N-oxide'. N,Ndimethyl-2- 1. 0 0. 63 O. 44

aminoethanol- N -oxide hydrate. 28-9 N,N-diethyl-2- 0.5 1.05 0.73

aminoethanol- N -oxide by drate. 28-10 Triethanolamine- 0.5 0.99 0.69

N-oxide hydrate. 29-1 None None 1. 45 29-2 N,N-dlmethyl-2- 2. 0 0. 72 0. 50

aminoethanol- N-oin'de hydrate. 29-'3 N,N-die'thyl-2- 2.0 0. 54 0.37

aminoethanol- N -oxide bydrate'. 30-4 None None 1.25 30-13 N,N-d1methy1- 1.0 0.77 0. 61

cyclohexylamine oxide. 325- None 'None 1.29 32-4 N,N-dimethyl-2- 0.5 0.60 -0. 46

amino-ethanol- N-oxide hydrate.

The solution of acetylene from the bottom of the absorber is introduced into a desorber where the pressure of the gas in contact with the solution is reduced with the result that the major portion of the less soluble non-acetylenic gases is flashed off. These less-soluble gases are returned to the scrubber for recovery of any acetylene contained in them. The acetylene-saturated solution of N-methyl pyrrolidone leaving the desorber passes through a heater wherein its temperature is raised and is then conducted into a series of stages or zones wherein the solvent is subjected to progressively lower pressures and whereby the gases dissolved in the solvent are liberated. The gases liberated in each stage or zone are removed from a lower pressure stage and injected into the next higher pressure stage by means of high velocity steam jets, the gas pressure in the stage of highest pressure being maintained sufliciently high so that there is a flow of gas from this stage to the bottom of the desorber. Pure concentrated acetylene is withdrawn from an intermediate section of the desorber. The liquid solvent from the bottom of the stage of lowest pressure substantially completely freed of acetylene is pumped to a solvent desorber where it is freed of remaining impurities and recycled to the absorption tower.

in this system where N,N-diethyl-Z-aminoethanol-N- oxide hydrate is incorporated in the solvent, polymerization of the unsaturated polymerizable materials in the gas being treated is held to a minimum. No appreciable amounts of polymer are deposited in the solvent or in the equipment thus obviating the usual processing difiiculties arising from plugged pipes, trays, etc.

The inhibitors of the invention are oxides of tertiary amines in which the substituents on the nitrogen atom may be alkyl, alkylol, aralkyl, aryl, cyclic or a combination of two or more of these groups and hydrates of such oxides. The compounds are derived from tertiary amines and a peroxide, preferably hydrogen peroxide. Examples of the difierent types of amine oxides which are operative are trialkyl amine oxides such as trimethyl, triethyl, tributyl, trioctyl amine oxides and the like; trialkylol amine oxides such as trimethanolamine oxide; mixed alkyl alkylol amine oxides such as diethyl ethanol amine oxide; triaryl amine oxides such as triphenyl amine oxide; mixed alkyl aryl amine oxides such as dirnethyl aniline oxide; triaralkyl amine oxides such as tribenzyl amine oxide; heterocyclic amine oxides such as pyridine oxide, quinoline oxide, and the like as well as the hydrates of all these oxides.

The eltectiveness of the inhibitors is not restricted to those conditions given in the examples. In general, only minor quantities of the inhibitors are required although the amount used depends somewhat on the amount of polyformers present in the gas stream and the degree of inhibition desired. Concentrations of the amine oxide or amine oxide hydrate in the range from about 0.01% to about 5% by weight of the solvent are suitable, with concentrations from about 0.1% to about 2% being preferred. The stabilizers or inhibitors may, of course, be employed in as great a concentration as desired.

The inhibitors of the invention are not restricted to use with only N-rnethyl pyrrolidone as set out in the examples above. They may be used to inhibit polymerization in other like organic solvents such as butyrolactone, for example. In any solvent system where a selective solvent is used for the recovery of acetylene from a gas stream containing it in combination with other gases including poly-formers by absorption in the solvent and subsequent stripping therefrom, polymerization of such poly-formers is effectively inhibited by the presence of a minor amount of an amine oxide or an amine oxide hydrate in the solvent. To inhibit polymer formation during the absorption-stripping operation, the inhibitors can be added directly to the solvent feed stream, or they can be introducedas a solution of the desired concentration at some point in the absorption system. The inhibiting power of the amine oxides and amine oxide hydrates is not affected by continual re-use; hence, recirculation of the solvent may be safely practiced with the inhibiting power of the additives of the invention suffering no significant decrease in effectiveness.

What is claimed is:

1. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with a solvent for selective absorption of acetylene therefrom and said solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating a compound from the group consisting of tertiary amine oxides and tertiary amine oxide hydrates in the solvent employed in an amount in the range from about 0.01% to about 5% by weight of the solvent.

2. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with a solvent for selective absorption of acetylene therefrom and said solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating a compound from the group consisting of tertiary amine oxides and tertiary amine oxide hydrates in the solvent employed in an amount in the range from about 0.1% to about 2% by weight of the solvent.

3. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating a compound chosen from the group consisting of tertiary amine oxides and tertiary amine oxide hydrates in said N-methyl pyrrolidone solvent in an amount in the range from about 0.01% to about 5% by weight of the solvent.

4. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating a compound chosen from the group consisting of tertiary amine oxides and tertiary amine oxide hydrates in said N-methyl pyrrolidone in an amount in the range from about 0.1% to about 2% by weight of the solvent.

5. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating N,N-dimethyl-2-aminoethanol-N-oxide hydrate in said N-methyl pyrrolidone solvent in an amount in the range from about 0.1% to about 2% by weight of the solvent.

6. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating N,N-diethyl-Z-aminoethanol-N-oxide hydrate in said N-methyl pyrrolidone solvent in an amount in the range from about 0.1% to about 2% by weight of the solvent.

7. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporat ing triethanolamine-N-oxide-hydrate in said N-methyl pyrrolidone solvent in an amount in the range from about 0.1% to about 2% by weight of the solvent.

8. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating N-rnethyl morpholine-N-oxide in said N-methyl pyrrolidone solvent in an amount in the range from about 0.1% to about 2% by weight of the solvent.

9. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating N,N-dimethyl cyclohexylamine-N-oxide in said N- methyl pyrrolidone solvent in an amount in the range from about 0.1% to about 2% by weight of the solvent.

References Cited in the file of this patent UNITED STATES PATENTS 2,846,443 Malusa et al Aug. 5, 1958 2,907,801 Johnson et al. Oct. 6, 1959 2,907,803 Rolland et al Oct. 6, 1959 

1. IN A PROCESS FOR THE RECOVERY OF ACETYLENE FROM AN ACETYLENE-CONTAINING GAS STREAM DERIVED FROM HYDROCARBONS WHEREIN THE ACETYLENE-CONTAINING GAS STREAM IS CONTACTEDWITH A SOLVENT FOR SELECTED ABSORPTION OF ACETYLENE THEREFROM AND SAID SOLUTION IS THEREAFTER STRIPPED TO RECOVER ACETYLENE THEREFROM, THE IMPROVEMENT WHICH COMPRISES INCORPORATING A COMPOUND FROM THE GROUP CONSISTING OF TERTIARY AMINE OXIDES AND TERTIARY AMINE OXIDE HYDRATES IN THE SOLVENT EMPLOYED IN AN AMOUNT IN THE RANGE FROM ABOUT 0.01% TO ABOUT 5% BY WEIGHT OF THE SOLVENT. 